1. Field of the Invention
This invention relates to an acid-soluble glass composition for use in manufacture of a flexible fiber optic bundle and to a process for manufacturing a flexible fiber optic bundle using the same.
2. Description of the Prior Art
A fiber optic bundle is formed of a multiplicity of optical fibers packed in tightly side-by-side bundled relation with each other. When the fiber optic bundle is used as an image-transmitting device, it is essential that the individual optical fibers be arranged in identical geometrical patterns at the opposite ends of the bundle so that each part of an image at the object end of the bundle will be reproduced at the image end thereof in the same location. In particular, when the fiber optic bundle is used as a flexible image-transmitting device such as an endoscope, it is further essential that the optical fibers are bonded to each other only at their opposite ends so that the optical fibers may be flexibly curved between their opposite ends. Various processes for manufacturing such a fiber optic bundle comprising the rigid end portions in which the optical fibers are bonded to each other and the flexible intermediate portion between the rigid end portions have heretofore been proposed.
For example, a process for manufacturing a flexible fiber optic bundle for image transmission comprising the following steps is known in the art;
(i) forming an optical fiber comprising a core of glass of relatively high refractive index and a coating of glass of relatively low refractive index surrounding the core,
(ii) winding up the optical fiber in the form of a spiral having no gaps between adjacent convolutions, bonding the adjacent convolutions to each other with an adhesive at a proper portion of the spiral, and repeating the winding up and the bonding in accumulated convolutions to obtain a loop-like bundle of the optical fibers having a desired width and thickness with a rigid portion in which the optical fibers are bonded to each other with the adhesive,
(iii) cutting the bundle laterally at the rigid portion, and
(iv) polishing the cut end faces of the bundle.
In the step (i), the optical fiber is formed, for example, by using a double-walled crucible having an orifice at the bottom thereof. That is, the glass of relatively high refractive index and the glass of relatively low refractive index are placed in the inner and the outer spaces of the double-walled crucible, respectively, and the crucible is heated to a proper temperature to melt the two kinds of glass. Then, the two kinds of molten glass are drawn downwardly simultaneously through the orifice to form an optical fiber.
In the above-mentioned process, the cross-sectional size of the optical fiber formed in the step (i) is not reduced during the subsequent steps (ii) to (iv) and accordingly, a very thin optical fiber having a diameter of 20.mu. for example is formed in the step (i) in order to increase the resolution of the image transmitted through the flexible fiber optic bundle. Accordingly, the operation to arrange the optical fibers, that is, the repeated winding up and bonding in the step (ii), demands operator's skill. In addition, there is a fear that the optical fibers are severed while being arranged. Therefore, the above-mentioned process has disadvantages that the reject rate of the flexible fiber optic bundle manufactured by the process is high and the manufacturing cost thereof is also high.
Another process for manufacturing a flexible fiber optic bundle in which acid leaching is utilized is also known in the art. The process comprises steps of;
(i) forming an optical fiber comprising a core of glass of relatively high refractive index, a first coating of acid-resisting glass of relatively low refractive index surrounding the core, and a second coating of acid-soluble glass surrounding the first coating,
(ii) bundling a multiplicity of the optical fibers (for example, ten thousand optical fibers) in side-by-side relation with each other to obtain a bundle of the optical fibers,
(iii) heating and drawing the bundle to form an elongated rigid bundle of reduced diameter, and
(iv) contacting the intermediate portion of the length of the elongated rigid bundle between the opposite end portions thereof with an acid such as nitric acid, and leaching the acid-soluble glass of the second coating from the intermediate portion to make the intermediate portion flexible.
The bundle of the optical fibers obtained after the step (ii) may be placed within a tubular casing of the same acid-soluble glass as that of the second coating. When the bundle of the optical fibers is placed within the tubular casing, the above steps (i) and (ii) are followed by steps of
(ii') placing the bundle within the tubular casing,
(iii) heating and drawing the bundle and the casing together to form an elongated rigid bundle of reduced diameter having the acid-soluble glass of the casing therearound, and
(iv) contacting the intermediate portion of the elongated rigid bundle between the opposite end portions thereof with an acid such as nitric acid, and leaching the acid-soluble glass of the second coating and of the casing from the intermediate portion to make the intermediate portion flexible.
In the above step (i), the optical fiber is formed, for example, by using a triple-walled crucible having an orifice at the bottom thereof. That is, the glass of relatively high refractive index, the acid-resisting glass of relatively low refractive index and the acid-soluble glass (the glass preferably has an coefficient of thermal expansion and viscosity similar to those of the former two glasses) are placed in the innermost, the next to the innermost and the outer spaces of the triple-walled crucible, respectively, and the crucible is heated to proper temperature to melt the three kinds of glass. Then, the three kinds of molten glass is drawn simultaneously through the orifice to form the optical fiber. The optical fiber thus obtained is cut into proper length (for example, about 300 mm) and a multiplicity of the optical fibers of a proper length obtained are bundled in the step (ii).
In the step (iv), the contact of the intermediate portion of the elongated rigid bundle with an acid is performed, for example, by coating the opposite end portions of the elongated rigid bundle with an acid-resisting material and then, immersing the whole elongated rigid bundle in an acid.
Differently from the aforementioned process, in the process utilizing acid leaching, the diameter of the optical fiber formed in the step (i) is reduced in the subsequent step (iii) and accordingly, a relatively thick optical fiber may be formed in the step (i). For example, an optical fiber having a diameter of about 200.mu. is formed in the step (i) and the optical fiber is elongated in the step (iii) to the extent that the diameter thereof is reduced to about 10.mu.. Accordingly, the operation of arranging the optical fibers in the step (ii) can be performed easily. Further, the optical fibers are hardly severed during the operation of arranging the optical fibers. Since the optical fibers are integrated when heated in the step (iii), the optical fibers are not severed in the subsequent steps (iii) and (iv) at all. Therefore, the reject rate of the flexible fiber optic bundle manufactured by the process utilizing acid leaching is lower than that of the flexible fiber optic bundle manufactured by the aforementioned process. Further, the manufacturing cost of the flexible fiber optic bundle manufactured by the process utilizing acid leaching is much lower than that of the flexible fiber optic bundle manufactured by the aforementioned process.
However, the process which utilizes acid leaching has the following disadvantage. That is, when the second coating of acid-soluble glass of the optical fiber is leached with an acid, the first coating of acid-resisting glass is also etched by the acid and as the result, the surface of the first coating is roughened. This is because the second coating of acid-soluble glass exists in extremely narrow gaps in the elongated rigid bundle. The resulting roughness of the surface of the first coating causes the severance of the optical fiber and accordingly, shortens the life of the bundle. The above problem can be solved by using an acid-soluble glass having sufficiently high acid-solubility in the second coating.
Such an acid-soluble glass is disclosed in Japanese Patent Publication No. 38623/1978 and U.S. Pat. No. 3,624,816. The acid-soluble glass disclosed in the former has a composition expressed by weight percent comprising about 45% of B.sub.2 O.sub.3, about 45% of BaO and about 8% of La.sub.2 O.sub.3 (when the composition is expressed by mol percent, the composition comprises about 65.7% of B.sub.2 O.sub.3, about 29.8% of BaO and about 2.5% of La.sub.2 O.sub.3). The acid-soluble glass disclosed in the latter has a composition expressed by weight percent comprising about 47% of B.sub.2 O.sub.3, about 45% of BaO and about 5% of La.sub.2 O.sub.3 (when the composition is expressed by mol percent, the composition comprises about 66.6% of B.sub.2 O.sub.3, about 28.9% of BaO and about 1.5% of La.sub.2 O.sub.3). However, the water resistance of the acid-soluble glass of the type disclosed in the above patents is extremely low though the acid-solubility thereof is sufficiently high. Accordingly, when the opposite ends of the flexible fiber optic bundle manufactured using the acid-soluble glass are polished, the acid-glass dissolves during the polishing. Because of the dissolution of the acid-soluble glass, it is difficult to polish the opposite ends. Further, since the BaO content of the acid-soluble glass of the type disclosed in the above patents is high, the variation in viscosity of the acid-soluble glass accompanying the variation in temperature is enlarged. Accordingly, it is very difficult to form an optical fiber having extremely high dimentional accuracy by using the acid-soluble glass.